Mail art is a movement in which small scale art works are sent through the postal service.
Mail art began in the 1960s when artists sent postcards inscribed with poems or drawings through the post rather than exhibiting or selling them through conventional commercial channels.
Its origins can be found in Marcel Duchamp and Kurt Schwitters and the Italian futurists. But it was the New York artist Ray Johnson who, in the mid 1950s, posted small collages, prints of abstract drawings and poems to art world notables giving rise to what eventually became known as the New York Correspondence School.
Source : TATE UK